| Proanthocyanidins are a group of polyphenols found in both malt and hops,
which in hops are often called tannins due to their greater complexity. They show affinity
towards certain proteins found in malt, namely hordein storage proteins, which are
characterized by a high content of proline residues. Although in malt the
proanthocyanidins are physically separated from these malt proteins, they are able to form
complexes during mashing and precipitation occurs. This process continues after wort
boiling and during fermentation, where the temperature is lowered and will occur in
bottled beer stored in the fridge if no precautions are taken. Effective precautions
include sufficient chilling after beer maturation and a subsequent stabilisation step: The interaction between tannins and hordein is based on the formation of hydrogen bonds which form more rapidly at lower the temperatures. Stabilisation consists of removing the tannins and/or the hordein from the beer. Proanthocyanidins and the complex tannins are removed with a synthetic insoluble polymer called polyvinylpolypyrrolidon (PVPP) which closely resembles polyproline. They both possess a carbonyl group neighbouring a proton free nitrogen atom, a very potent site for the formation of hydrogen bonds. Alternatively hordein may be removed by treatment with specific silica gels, hydrogels or xerogels. An even better stability is obtained when reactants are removed or partially removed. Another approach to brew colloidally stable beer is to use proanthocyanidin-/tannin-free raw materials. Tannin-free hop extracts are available world-wide and at Carlsberg intensive research over the last 25 years has resulted in the breeding of acceptable proanthocyanidin free malting barley. So in the future stabilisation may become superfluous. |
Figure 1: Two dimeric proanthocyanidins in barley/malt.
Figure 2: Two glasses of beer: a. Colloidally stable beer
Figure 3: Schematic formulation of PP and PVPP. |
